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Applied Physics Letters

Issue 16 • Date Apr 2004

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Displaying Results 1 - 25 of 82
  • Issue Cover

    Page(s): c1
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    Freely Available from IEEE
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Microscopic quantum theory of spatially resolved photoluminescence in semiconductor quantum structures

    Page(s): 2971 - 2973
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    We present a microscopic analysis of spatially resolved photoluminescence and photoluminescence excitation spectroscopy in semiconductor quantum structures. Such theoretical and numerical framework provides a general basis for the description of spectroscopic imaging in which the excitation and detection energies and spatial positions can all independently be scanned. The numerical results clarify the impact of the near-field optical setup on the obtained images and resolutions. © 2004 American Institute of Physics. View full abstract»

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  • Continuous-wave second-harmonic generation in modal phase matched semiconductor waveguides

    Page(s): 2974 - 2976
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    We report the observation of continuous-wave second-harmonic generation in a modalphase-matched GaAs/AlGaAs waveguide. The heterostructure has been designed to generate a second-harmonic signal via a type-II process with a fundamental signal around 1.55 μm. Continuous wave conversion enables a quantitative estimation of the conversion efficiency. In our case, η=30%±5% W-1cm-2 is found experimentally. © 2004 American Institute of Physics. View full abstract»

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  • Middle-infrared transmission enhancement through periodically perforated metal films

    Page(s): 2977 - 2979
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    Experimental results of extraordinary optical transmission through periodically perforated metal films in the middle-infrared region are reported. The position of the transmission pass bands depends slightly on the aperture size but mainly on the periodicity. The transmission enhancement, due to the involvement of surface plasmon polaritons, can be improved by forming a symmetrical structure. © 2004 American Institute of Physics. View full abstract»

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  • Photoinduced partial charge transfer between conjugated polymer and fullerene in solutions

    Page(s): 2980 - 2982
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    Photoinduced charge transfer between a conjugated polymer and C60 and the related processes were investigated in dilute solutions. The substantial fluorescence quenching is correlated with the efficient exciton diffusion within the polymer chains, according to which a sphere-of-action mechanism is proposed. An emissive exciplex was found formed between the conjugated polymer and fullerene in a nonpolar solvent, indicating the occurrence of a photoinduced partial charge transfer process. The low-energy sites in the polymer are believed to play a crucial role in the partial charge transfer. The asymmetry of the exciplex provides a method for evaluating the tendency of photoinduced charge separation between the donor and the acceptor. This method allows screening candidates for photovoltaic applications. © 2004 American Institute of Physics. View full abstract»

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  • High-efficiency chemical oxygen–iodine laser using a streamwise vortex generator

    Page(s): 2983 - 2985
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    A supersonic expansion nozzle has allowed us to achieve highly efficient operation of the supersonic mixing chemical oxygen–iodine laser (COIL). The nozzle’s shape produced a rapid mixing of the primary and secondary flows. Made up of a series of thin alternating wedges placed adjacent to each other, the nozzle looks like the letter X when it is viewed from the side. Iodine is injected at the exit plane of the nozzle and is strongly entrained by the streamwise vortices generated by the nozzle. 599 W of output power with a chemical efficiency of 32.9% is obtained, values higher by a factor of 1.4 than those of the conventional COIL. © 2004 American Institute of Physics. View full abstract»

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  • Light outcoupling efficiency of top-emitting organic light-emitting diodes

    Page(s): 2986 - 2988
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    We report results obtained from modeling the light outcoupling efficiency of top–emitting organic light-emitting diode (OLED) structures and compare them with results from conventional substrate-emitting structures. We investigate two types of emissive material, small molecule and conjugated polymers, and study three different cathode materials; aluminum, silver, and calcium. We show that top-emitting OLEDs may have outcoupling efficiencies comparable to their substrate-emitting counterparts, and that the choice of cathode material is critical to the optical performance of the device. © 2004 American Institute of Physics. View full abstract»

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  • Gradual facet degradation of (Al,In)GaN quantum well lasers

    Page(s): 2989 - 2991
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    In our study, III-nitride laser diodes with uncoated facets obtained by cleavage show a much faster degradation than coated ones. An increase in threshold current and drop of slope efficiency suggest increased absorption losses. Degradation experiments in different atmospheres prove the influence of the respective atmosphere and indicate the growth of an oxide film leading to increased absorption. Because the observed degradation is insensitive to the photon density we suggest nonradiative centers, which are saturated at low photon densities, to be at the origin of degradation. No evidence for photon enhanced degradation of coated laser diodes was found. A dielectric coating efficiently protects the facets. © 2004 American Institute of Physics. View full abstract»

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  • 1.24 μm InGaAs/GaAs quantum dot laser grown by metalorganic chemical vapor deposition using tertiarybutylarsine

    Page(s): 2992 - 2994
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    Metalorganic chemical vapor deposition of GaAs-based laser diodes, using self-organized InGaAs quantum dots (QDs), emitting at ≫1.24 μm is demonstrated. The environment-friendly alternative precursor tertiarybutylarsine is used as a substitute for arsenic hydride. The active region contains ten closely stacked InGaAs QD layers embedded in a GaAs matrix. Lasing emission at such long wavelengths was achieved by overgrowing the In0.65Ga0.35As QDs with a thin In0.2Ga0.8As film. The application of an in situ annealing step leading to the evaporation of plastically relaxed defect clusters is shown to be decisive for the laser performance. A transparency current density of 7.2 A/cm2 per QD layer and an internal quantum efficiency of 75% were achieved at room temperature. © 2004 American Institute of Physics. View full abstract»

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  • Simulation and design of GaN/AlGaN far-infrared (λ∼34 μm) quantum-cascade laser

    Page(s): 2995 - 2997
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    Designs of GaN/AlGaN quantum-cascade lasers emitting at 34 and 38 μm (ΔE∼36 and 34 meV) are presented, assuming either a- or c-plane crystal growth orientation. In the calculation of the quasibound state energies and wave functions, we account for the intrinsic electric field induced by piezoelectric and (in case of c-plane growth) the spontaneous polarization. The quantum-cascade structures were simulated, and their output characteristics extracted, using a fully self-consistent rate equation model with all relevant intra- and interperiod scatterings included. Both electron–LO-phonon and electron–electron scattering mechanisms are taken into account. Maximal population inversions between active laser states of up to 19% for the a-plane, and up to 40% for the c-plane design, are predicted and, based on estimated modal gain and waveguide/mirror losses in suitably designed structures, these indicate the feasibility of laser action in GaN/AlGaN cascades. © 2004 American Institute of Physics. View full abstract»

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  • Room-temperature optically pumped laser emission from a-plane GaN with high optical gain characteristics

    Page(s): 2998 - 3000
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    Photoluminescence (PL) and optical gain (OG) spectra of a-plane GaN layers have been analyzed over a wide range of excitation intensities. The samples were fully coalesced layers grown by metalorganic chemical vapor deposition over r-plane sapphire substrates using epitaxial layer overgrowth (ELOG) and selective area lateral epitaxy (SALE) procedures. ELOG and SALE a-plane samples showed a strong stimulated emission line in backscattering-geometry PL spectra along with extremely high OG coefficient values (in SALE samples more than 2000 cm-1). Structures prepared with natural cleaved facet cavities based on these films were used to demonstrate optically pumped room-temperature lasing. © 2004 American Institute of Physics. View full abstract»

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  • Laser frequency stabilizations using electromagnetically induced transparency

    Page(s): 3001 - 3003
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    We present two techniques to lock a laser frequency on an atomic transition line by using an electromagnetically induced transparency (EIT) signals, which give optical and electric feedback. We employed these methods to the D2 line of 87Rb atoms. Spectral characteristics of subnatural linewidth EIT allow us to improve frequency stability. By optical feedback of EIT signals, we were able to obtain locking bandwidths of 12 MHz, while frequency stability was 5×10-13 at best after 100 s of integration using a frequency modulation technique. © 2004 American Institute of Physics. View full abstract»

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  • Analytic modeling for the limit of mask mean-to-target from the photolithographic standpoint

    Page(s): 3004 - 3006
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    The critical dimension (CD) deviation from a nominal CD induced by the mask mean-to-target (MTT) is normally compensated by adjusting the exposure dose. However, the compensation is not accomplished to both cell and peripheral patterns which have different pitch sizes. In general, the exposure dose is adjusted to obtain the nominal CD of the cell pattern which has smaller pitch size than the peripheral pattern. The final CD of the cell pattern recovers its nominal CD by changing the exposure dose while that of the peripheral pattern does not but the final CD of the peripheral pattern should be located within a given tolerance range. Based on this idea, an analytic model for the limits of the mask MTT is proposed in this letter. Mask error enhancement factors and exposure latitudes for an island and two types of line and space patterns are obtained experimentally. Then, arguments adopted to derive the analytic expression for the MTT are assessed by comparing measured peripheral CDs with calculated ones for three types of patterns. Results are shown to match within 2% of nominal CDs under the different mask MTTs of 13.1, 19.7, and 26.2 nm at the mask scale (4×). The mask MTT specifications for three types of patterns are calculated at the CD tolerances of ±4 nm. The calculated limit of the mask MTT is ±4.3 nm for the island pattern. ±9.8 nm and ±9.3 nm of mask MTT limits are obtained for two types of line and space patterns. © 2004 American Institute of Physics. View full abstract»

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  • Deep-ultraviolet emission from an InGaAs semiconductor laser

    Page(s): 3007 - 3009
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    It is reported that a normal InGaAs laser diode (LD) operating at 980 nm possesses a second harmonic at 490 nm and a strong deep-UV emission at room temperature. By comparing with the radiation from cadmium sulphide (CdS) nanoparticles and nanowires, it is found that the UV emission from the LD can be attributed to a characterized radiation of nanoscaled semiconductors. By doping different semiconductor nanomaterials into the active layers of the LD and taking some effective techniques, such as cavity optimization, the wavelength, and the power output of the UV emission could be adjusted. © 2004 American Institute of Physics. View full abstract»

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  • Noncollinear type-II second-harmonic generation in a Al(0.3)Ga(0.7)As/Al2O3 one-dimensional photonic crystal

    Page(s): 3010 - 3012
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    We demonstrate noncollinear type-II second-harmonic generation in one-dimensional photonic crystals. A 15-period Al(0.3)Ga(0.7)As/Al2O3 structure, 3.5 μm long, was designed, fabricated, and experimentally characterized. We measured an effective nonlinearity of (52±12) pm/V in perfect phase-matching conditions. © 2004 American Institute of Physics. View full abstract»

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  • 4.2% efficient organic photovoltaic cells with low series resistances

    Page(s): 3013 - 3015
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    We demonstrate double-heterostructure copper phthalocyanine/C60 organic photovoltaic cells with series resistances as low as 0.1 Ω cm2. A high fill factor of ∼0.6 is achieved, which is only slightly reduced at very intense illumination. As a result, the power conversion efficiency increases with the incident optical power density, reaching a maximum of (4.2±0.2)% under 4–12 suns simulated AM1.5G illumination. The cell performance is accurately described employing an analysis based on conventional semiconductor p–n junction diodes. The dependence of the series resistance on the device area suggests the dominance of the bulk resistance of the indium-tin-oxide anode as a limiting factor in practical cell efficiencies. © 2004 American Institute of Physics. View full abstract»

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  • Slab-edge modes in two-dimensional photonic crystals

    Page(s): 3016 - 3018
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    We demonstrate the existence of surface waves in two-dimensional photonic crystal slab edges. In finite-sized air-terminated photonic crystal boundaries, the slab-edge mode turns into a lasing mode under pulsed optical pumping conditions. Analyses of the modal behaviors of the slab-edge modes by means of calculations based on the plane wave expansion method and the finite-difference time-domain method show that resonant frequencies and quality factors are strongly dependent on the termination parameter and the shape of the slab corner. © 2004 American Institute of Physics. View full abstract»

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  • Polymer gratings with low surface relief based on photopolymerization-induced internal diffusion

    Page(s): 3019 - 3021
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    A photopolymer formulation sensitive to 325 nm ultraviolet light is proposed for the fabrication of polymer gratings based on photopolymerization-induced internal diffusion process. A very low surface relief depth ranging from 12.4 to about 1.0 nm has been demonstrated with a refractive index modulation Δn of about 0.010. Such polymer gratings show promising potentials for the fabrication of low-order distributed feedback polymer lasers. © 2004 American Institute of Physics. View full abstract»

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  • Tunable distributed feedback color center laser using stabilized F2+** color centers in LiF crystal

    Page(s): 3022 - 3024
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    We report a room temperature tunable color center distributed feedback (DFB) laser using stabilized F2+** centers in LiF (LiF:F2+**) as a gain medium. Tunable oscillation of LiF:F2+** DFB lasing was achieved in the near infrared region (882–962 nm) with a lasing linewidth of less than 0.2 cm-1. Also the lasing threshold and slope efficiency with respect to pump energy were found to be 1.2 mJ and up to 3%, respectively. © 2004 American Institute of Physics. View full abstract»

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  • Phase locking of a two-dimensional laser array by controlling the far-field pattern

    Page(s): 3025 - 3027
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    We report phase locking of a two-dimensional laser array through beam control in the far field of the emitters. Compared to the near-field techniques, the far-field approach involves simpler beam profiles and does not rely on having a large number of emitters in a perfectly periodic structure. Our experimental results and computer simulation also point to much higher feedback efficiency, less sensitivity to power variations among the emitters, and the ability to repair a missing element. © 2004 American Institute of Physics. View full abstract»

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  • Temperature gradient in Hall thrusters

    Page(s): 3028 - 3030
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    Plasma potentials and electron temperatures were deduced from emissive and cold floating probe measurements in a 2 kW Hall thruster, operated in the discharge voltage range of 200–400 V. An almost linear dependence of the electron temperature on the plasma potential was observed in the acceleration region of the thruster both inside and outside the thruster. This result calls into question whether secondary electron emission from the ceramic channel walls plays a significant role in electron energy balance. The proportionality factor between the axial electron temperature gradient and the electric field is also significantly smaller than might be expected by fluid models. © 2004 American Institute of Physics. View full abstract»

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  • The role of the InGaAs surface in selective area epitaxy of quantum dots by indium segregation

    Page(s): 3031 - 3033
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    The surface of strained InGaAs films for selective regrowth of InAs nanostructures is investigated by atomic force microscopy and Rutherford backscattering. 3.3-nm-thick In0.33Ga0.67As films were annealed at temperatures between 400 and 800 °C. Significant indium desorption was found to occur at temperatures above 550 °C. The optimum parameters are presented for selective growth of InAs quantum dots having densities of 6.6×1010cm-2 on In0.33Ga0.67As films. © 2004 American Institute of Physics. View full abstract»

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  • Nanothermodynamic analysis of the low-threshold-pressure-synthesized cubic boron nitride in supercritical-fluid systems

    Page(s): 3034 - 3036
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    A thermodynamic model in nanoscale was developed to elucidate the nucleation of the cubic boron nitrides (c-BN) synthesized in the high-pressure and high-temperature (HPHT) supercritical-fluid systems under the conditions of the low-threshold-pressure (from 2.0±0.1 to 3.0 GPa) and low-temperature (1300–1500 K). Notably, taking the nanosize-induced interior pressure into account, the nucleation of c-BN could be driven to the new stable region from the metastable region in the general accepted equilibrium phase (P,T) diagram of BN proposed by Corrigan and Bundy, upon HPHT supercritical-fluid systems. The threshold pressure of the formation of c-BN was calculated based on our model, and these results are in excellent agreement with the experiment. © 2004 American Institute of Physics. View full abstract»

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  • Visible light emission from polymer-based field-effect transistors

    Page(s): 3037 - 3039
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    Field-effect transistors (FETs) based on poly [2-methoxy, 5-(2-ethyl-hexoxy)-1, 4-phenylenevinylene] (MEH-PPV) were prepared with bottom-contact type interdigital electrodes of Cr/Au and Al/Au on the SiO2/Si substrates. MEH-PPV exhibited a p-type semiconducting behavior and orange light emission was observed when the devices were operated in vacuum. It was found that the luminescence efficiency of the FETs with Al/Au electrodes was higher than that of Cr/Au electrodes. The simultaneous injection of holes and electrons into MEH-PPV occurred efficiently with the application of Al/Au heteroelectrodes. © 2004 American Institute of Physics. View full abstract»

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Applied Physics Letters, published by the American Institute of Physics, features concise, up-to-date reports on significant new findings in applied physics.

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Nghi Q. Lam
Argonne National Laboratory